The present invention relates generally to determining airway resistance and lung compliance, and more particularly to using a circuit model approach to find the airway resistance and lung compliance where at least one component is non-linear.
In an anesthesia procedure, it is advantageous to know with some certainty the airway resistance and lung compliance in order to ascertain the suitability of the ventilatory management system. Although many attempts have been made to determine airway resistance, most assume the resistance to be linear, when in fact, it is not. Others do not consider the effects of lung compliance. Therefore, both types of systems fall far short of determining either parameter with any certainty and in some cases, results in gross inaccuracies.
It will be shown herein that those systems that assume a linear resistance relationship between pressure and flow in the patient airway will not function properly on intubulated patients. It is also believed that since an endotracheal tube follows the natural path and shape of a patient's airway, such linear systems will not function properly if applied directly to the patient's airway passage. The error found in the results from such linear techniques for determining resistance and compliance will increase dramatically with varying ventilation flow rates. Changing flow rates is common in anesthesia procedures and can be caused by changes to ventilatory settings of tidal volumes, inspiratory flows, or fresh gas delivery to the breathing circuit. In order to assume such a linear relationship then, one must maintain a constant flow rate. However, such an undesirable dependence on requiring a constant flow rate, can also impose errors to the very parameters being estimated because flow rate changes during each breath cycle as well.
Some prior art systems require the injection of an excitation flow into the breathing circuit or an inspiratory pause in order to calculate the airway resistance and lung compliance. However, such techniques are not practical during an anesthesia procedure. In fact, in any breathing system where fresh gas is supplied constantly from a gas source other than the ventilator, an inspiratory pause cannot be imposed.
Other known systems use a forced high frequency oscillation to determine airway resistance and lung compliance. The problem with this system is that no one good resonant frequency can be determined for all patients. Attempting to find the correct frequency for each patient would be time consuming and not practical.
One early attempt at determining lung airway resistance non-linearly is disclosed in U.S. Pat. No. 3,036,569. However, merely finding a resistance at one flow rate does not provide sufficient data to be useful in anesthesia procedures. It has also been found that pressure is not a function of resistance only, but also of compliance. Further, this reference requires an apparatus that forces air into the lung in order to perform the calculation, which would interfere with normal breathing and with anesthesia flow.
It would therefore be desirable to have a system, including a method and apparatus, that does not interfere with normal breathing, is non-intrusive with the normal flow and pressure during an anesthesia procedure, can measure pressure and flow on expiration, does not interrupt or interfere in any way with the respiratory pattern, and can find both airway resistance and lung compliance, while still reporting the non-linear air resistance at a standardized flow rate.